In recent years, the advance of electronic technology has realized an improvement in the performance, size reduction and portable structures of a variety of electronic apparatuses. Also secondary batteries for use as the power sources for the electronic apparatuses must have high performance and reduced sizes.
As a new secondary battery which is capable of satisfying the foregoing requirements, a non-aqueous electrolyte secondary battery is expected to be employed. In particular, lithium-ion secondary batteries, which are able to charge and discharge lithium-ion, have energetically been developed and put into practical use.
A positive-electrode active material for the foregoing battery has mainly been any one of vanadium pentoxide, manganese oxide, lithium cobaltate acid, lithium nickelate acid and spinel type lithium manganate. The negative electrode is made of a carbon material, such as graphite. Thus, a high-voltage and high-energy-density battery has been put into practical use.
A battery of the type made of the above-mentioned materials, however, is unfit for use in a low voltage region not higher than 3 V.
It can be considered that reduction in the voltage required for each of the apparatuses will furthermore be attempted because of the advancement of the technology for developing the electronic apparatuses and reduction in the voltage required to operate ICs.
Under the foregoing circumstances, a battery incorporating a negative electrode containing spinel type lithium titanate has been combined with a solar cell so as to be put into practical use as a power source for a watch, which is operated at a voltage of about 1.5 V. The future trend of the above-mentioned combination in the market has received attention (see "Lithium-Ion Titanate Secondary Battery" Document No. N03-04, New Material, August 1996).
The above-mentioned battery system has an excellent cycle characteristic, over discharge resistance and a preservation characteristic. It can be considered that requirements for development of batteries having larger charging/discharging capacities will be imposed if 1.5 V batteries will be used more widely.
Hitherto, the characteristics of titanium oxide, which is a raw material of lithium titanate for forming a positive and negative electrode of a non-aqueous electrolyte secondary battery as an active material, exert a considerable influence on the characteristics of the battery. Therefore, research and development have energetically been reported.
Titanium oxide, which is the raw material, has some structure of brookite, rutile, anatase and so forth. A consideration is made that anatase titanium oxide is an advantageous raw material because of further excellent charging/discharging performance and a satisfactory cycle characteristic of the battery made from it.
As a known lithium compound advantageous as the material of the lithium-ion secondary battery, lithium titanate is known which is formed into any one of Li.sub.2.67 Ti.sub.1.33 O.sub.4, LiTi.sub.2 O.sub.4, Li.sub.1.33 Ti.sub.1.66 O.sub.4, Li.sub.1.44 Ti.sub.1.71 O.sub.4, Li.sub.0.8 Ti.sub.2.2 O.sub.4 and the like. Among the foregoing lithium compounds, Li.sub.1.33 Ti.sub.1.66 O.sub.4 has attracted attention because of its large charging/discharging capacity.
To obtain lithium titanate, a method has been employed in which a mixture of titanium oxide and a lithium compound is subjected to dry heat treatment at temperatures of 700.degree. C. to 1600.degree. C. (see, Japanese Patent Laid-Open No. 6-275263).
Thus-obtained mission lithium titanate Li.sub.1.33 Ti.sub.1.66 O.sub.4 has a theoretical charging/discharging capacity of 175 mAh/g in one electronic reaction. However, materials commercially available at present have a charging/discharging capacity of about 130 mAh/g to about 150 mAh/g in 0.75 electronic reaction to 1.0 electronic reaction. That is, the value is excessively smaller than the theoretic capacity (see, Electrochemistry, p.p. 870 to 875, vol. 62, No. 9 (1994)).
In recent years, notebook personal computers, portable CD players, MD players and so forth have widely been used. Therefore, secondary batteries, which are mounted as the power sources of the foregoing apparatuses, are considerably required. Moreover, size reduction causes a requirement for a longer operating time and longer lifetime to arise. Therefore, a material having a large charging/discharging capacity is required considerably.